Several types of spear guns are known for use underwater and may generally be catagorized by the type of propulsion employed for the spear. For example, one type of gun uses one or more elastic cords fixed near the tip of the gun. The cords are stretched and hooked to a spear carried by the gun. When the spear is released, the elastic cords rapidly contract and propel the spear. Another type of gun is an air powered gun which uses compressed air to force a piston down a smooth barrel. A spear resting within the barrel is propelled by the piston as the piston travels down the barrel.
Another type of gun uses a conventional firearm cartridge to propel a spear. One such gun uses a .38 caliber firearm cartridge including a projectile and a primer in one end of the cartridge to ignite the gun powder therein. Other examples of a conventional firearm cartridges include a "blank type" firearm cartridge disclosed in U.S. Pat. No. 3,616,561 issued to G. E. Hendricks and a conventional .22 caliber cartridge used to propel a dart into a target as disclosed in U.S. Pat. No. 3,838,532 to Prodanovich. Lastly, so-called "bang sticks" used to repell sharks and the like employ a small waterproof shotgun-like cartridge which expells pellets and a burst of expanding gases toward a shark.
Each of the cartridges just mentioned have distinct drawbacks and disadvantages. The .38 caliber cartridge, the .22 caliber cartridge, and the "bang stick" cartridge each include projectiles which would be damaging to the internal structure of a spear gun. Furthermore, the .38 caliber, .22 caliber, and "blank type" cartridges are not water tight, leading to water saturated powder and the likelihood of misfires and the resulting failure of the spear gun.